I figured out a sure-fire way to get it to work. Right Click on the face of the Youtube video and select Copy Video URL. Create or edit the post you want to embed the video in. Provide a brief description. Space down a couple of lines then click on the Movie Film Icon above the text box and in the URL box paste what you copied in that box and then click Embed. Then click on Save Changes if editing, Post Reply or Create New Thread if creating a new post.
Keith, Actually, Xenforo has a simpler way to add the videos. Just paste the share URL. That's all you have to do. In other words, copy the share URL from YouTube and past it into a post here. The software does the rest. Charlie